1. Field of the Invention
The present invention generally relates to time division duplex indoor wireless communication networks and, more particularly, to a frequency look-ahead, packet/slave scheduling scheme and master/slave link characterization using a link state history table in the master unit in order to account for channel and system characteristics.
2. Background Description
Bluetooth™ is a computing and telecommunications industry specification that describes how mobile phones, computers, personal digital assistants (PDAs), and other devices can interconnect using a short range wireless connection. Each device is equipped with a microchip transceiver that transmits and receives in the frequency band of 2.45 GHz. Each device will have a unique 48-bit address from the IEEE 802 standard. Connections are one-to-one, and the maximum range is ten meters. Data can be exchanged at a rate of one megabits per second (Mbs) and up to two Mbs in the second generation of the technology. The five founding companies of the Bluetooth™ Special Interest Group (SIG) are Ericsson, IBM, Intel, Nokia, and Toshiba. Additional information may be had by reference to the Web site www.bluetooth.com and an article by Andrew Seybold entitled “Bluetooth Technology: The Convergence of Communications and Computing”, reprinted from Andrew Seybold's Outlook, May 1998, on the World Wide Web at www.gsmdata.com/artblue.htm.
Indoor wireless networks based on standards such as Bluetooth™ use frequency hopping to combat the problem of interference from sources such as microwave ovens and cordless telephones, which also use frequencies in the same band. In practical environments, in addition to active interfering sources, there can also be objects such as water fountains and racks of bottles with water content which absorb much of the radiation in the 2.45 GHz band and obstruct communication between master and slave units in the vicinity. Therefore, a master unit needs to detect such problems in communication and take necessary actions to prevent loss of packets during the periods of interference.
In the current Bluetooth™ standard, due to frequency hopping, the carrier frequency used in consecutive time slots is a different one of several different frequencies within the 2.45 GHz band of frequencies. Therefore, an interference in sub-bands centered around one of these frequencies will only affect communication during that time-slot in which the frequency sub-band is used. Further, in the Bluetooth™ standard, a packet can occupy one, three or five time slots, and in the case of multiple size packets, the same frequency as fixed for the first time slot is used. Because of this, it is possible to mask the effect of an interference by transmitting a packet of appropriate size. For example, if it is known that there is high chance of interference in one of the second through fifth time slots, and very low probability of the first time slot being bad, it is possible to skip the frequencies corresponding to second through fifth time slots by transmitting a five time-slot packet instead of one or three time-slot packets.
The characterization of link between any slave unit and the master can be done by the master unit based on the receipt or otherwise of acknowledgments received from the slave. Alternatively, all the slaves can record the number of times they detect good packet headers sent by the master to any slave. This information can be transmitted from the slaves to the master at periodic intervals of time. The master can use this information along with frequency look-ahead to determine the next slave for communication and also the appropriate packet size.
Several methods and schemes to combat the effect of interference in cellular wireless communication systems have been proposed. For example, a Fast Fourier Transform (FFT) based adaptive interference cancellation method has been proposed in U.S. Pat. No. 5,612,978 to Blanchard et al for “Method and Apparatus for Real-time Adaptive Interference Cancellation in Dynamic Environments”. The method allows relatively fast changes in the interference environment to be tracked and rejected.
In the U.S. Pat. No. 5,541,954 to Emi et al. for “Frequency Hopping Communication Method and Apparatus Changing a Hopping Frequency as a Result of a Counted Number of Errors”, a technique of changing the hopping frequency based on the number of errors encountered on a given frequency is proposed. This scheme is suitable in systems where the hopping frequency can be changed at any time.
Another related invention can be found in U.S. Pat. No. 5,323,447 to Gillis et al. for “Apparatus and Method for Modifying a Frequency Hopping Sequence of a Cordless Telephone Operating in a Frequency Hopping Domain”, in which the hopping sequence is modified for a cordless telephone. Substitute alternative communication channels are identified and then substituted for those communication channels experiencing interference. This is done without disrupting the communications between the handset unit and its associated base unit.
In systems in which the hopping frequency is fixed depending on the frequency hopping sequence corresponding any given pico-cell, there is need for other methods to combat the problem of interference. In U.S. Pat. No. 5,570,352 to Pöyhönen for “Digital Cellular Network/System with Mobile Stations Communicating with Base Stations Using Frequency-Hopping and Having Enhanced Effect of Interference Diversity”, frequency hopping utilizing interference diversity is presented.
In U.S. Pat. No. 5,659,879 to Dupuy for “Method of Covering Shadow Areas in a Cellular Mobile Radio and Radio Booster for Implementing this Method”, the problem of covering shadow areas by using radio boosters is addressed. To cover the shadow areas, a radio signal received by a radio booster from the base transceiver station on a basic frequency is re-transmitted to a mobile station on a translated frequency different from the basic frequency and associated with the latter by a translation law.